1. Field of the Invention
The present invention relates to sputtering apparatus for the deposition of thin films of a material other than metals on substrates. More particularly, it relates to a structure of a target installation of the sputtering apparatus, which prevents contamination of the thin films resulting from thermal breakage of a porous or poor heat-conducting target of a material other than metals during sputtering.
2. Description of the Prior Art
There are many sputtering systems such as, for example, diode DC sputtering systems, triode DC sputtering systems, radio-frequency sputtering systems, which are used for producing a thin film on a substrate. In all these sputtering systems, when producing thin films of a material other than metals it is conventional to mount a target of a film-forming material on a cathode of a sputtering system by means of fittings, thereby making the target and the cathode come closely into contact. However, a target installation of such structure has serious problems awaiting a solution. For example, in the radio-frequency sputtering systems the rate of forming thin films increases with an increase of the amount of the radio-frequency electric power supplied to the target so that it is customary to supply as much electric power as possible to increase the film-forming rate. However, the supply of a large amount of electric power frequently causes breakage of the target when the target is of a material other than metals such as, for example, ceramics, glasses, resins, sintered alloys and the like. The breakage of the target results in contamination of the thin films because the cathode is impacted by ions which are produced by a gaseous discharge and passed through cracks of the target, thereby ejecting the atoms of the cathode material.
It appears that one of the reasons why breakage of the target occurs during sputtering is the difference in temperature between the cathode and the target which occurs during sputtering for the following reasons. During sputtering, the target generates heat as a result of ion-bombardment, while the cathode is cooled by cooling water. Since the target of the above material is porous or poorly heat-conducting, its temperature is considerably increased. Accordingly, increasing the electricity supplied to such a porous or poor heat-conducting target results in a large temperature differential between the target and the cathode. Furthermore, the heat generated by the ion-bombardment causes the target to expand so that the target breaks when its thermal expansion force is greater than its tensile strength.
In order to overcome the above problem, it has been proposed to reinforce the porous or poor heat-conducting target by placing a solid reinforcing layer between the target and the cathode, which is formed by coating a paste or mixture of a binder and ceramic or glass powder on one surface of the target to be in contact with the cathode, and then heat-treating the paste. However, it is difficult with such a reinforcing layer to avoid breakage of the target.